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94 lines
5.0 KiB
Markdown
94 lines
5.0 KiB
Markdown
# PMTiles version 3
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## File structure
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A PMTiles archive is a single-file archive of square tiles with five main sections:
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1. A fixed-size, 127-byte **Header** starting with `PMTiles` and then the spec version - currently `3` - that contains offsets to the next sections.
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2. A root **Directory**, described below. The Header and Root combined must be less than 16,384 bytes.
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3. JSON metadata.
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4. Optionally, a section of **Leaf Directories**, encoded the same way as the root.
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5. The tile data.
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## Entries
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A Directory is a list of `Entries`, in ascending order by `TileId`:
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Entry = (TileId uint64, Offset uint64, Length uint32, RunLength uint32)
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* `TileId` starts at 0 and corresponds to a cumulative position on the series of square Hilbert curves starting at z=0.
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* `Offset` is the position of the tile in the file relative to the start of the data section.
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* `Length` is the size of the tile in bytes.
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* `RunLength` is how many times this tile is repeated: the `TileId=5,RunLength=2` means that tile is present at IDs 5 and 6.
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* If `RunLength=0`, the offset/length points to a Leaf Directory where `TileId` is the first entry.
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# Directory Serialization
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Entries are stored in memory as integers, but serialized to disk using these compression steps:
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1. A little-endian varint indicating the # of entries
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2. Delta encoding of `TileId`
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3. Zeroing of `Offset`:
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* `0` if it is equal to the `Offset` + `Length` of the previous entry
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* `Offset+1` otherwise
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4. Varint encoding of all numbers
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5. Columnar ordering: all `TileId`s, all `RunLength`s, all `Length`s, then all `Offset`s
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6. Finally, general purpose compression as described by the `Header`'s `InternalCompression` field
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# Directory Hierarchy
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* The number of entries in the root directory and leaf directories is up to the implementation.
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* However, the compressed size of the header plus root directory is required in v3 to be under **16,384 bytes**. This is to allow latency-optimized clients to prefetch the root directory and guarantee it is complete. A sophisticated writer might need several attempts to optimize this.
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* Root size, leaf sizes and depth should be configurable by the user to adjust for optimize for different trade-offs: cost, bandwidth, latency.
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# Header Design
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*Certain fields belonging to metadata in v2 are promoted to fixed-size header fields. This allows a map container to be initialized to the desired extent or center without blocking on the JSON metadata.*
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The `Header` is 127 bytes, with little-endian integer values:
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| offset | description | width |
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| --- | --- | --- |
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| 0 | magic number `PMTiles` | 7 |
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| 7 | spec version, currently `3` | 1 |
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| 8 | offset of root directory | 8 |
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| 16 | length of root directory | 8 |
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| 24 | offset of JSON metadata, possibly compressed by `InternalCompression` | 8 |
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| 32 | length of JSON metadata | 8 |
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| 40 | offset of leaf directories | 8 |
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| 48 | length of leaf directories | 8 |
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| 56 | offset of tile data | 8 |
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| 64 | length of tile data | 8 |
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| 72 | # of addressed tiles, 0 if unknown | 8 |
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| 80 | # of tile entries, 0 if unknown | 8 |
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| 88 | # of tile contents, 0 if unknown | 8 |
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| 96 | boolean clustered flag, `1` if true | 1 |
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| 97 | internal compression enum (0 = Unknown, 1 = None, 2 = Gzip, 3 = Brotli, 4 = Zstd) | 1 |
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| 98 | tile compression enum | 1 |
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| 99 | tile type enum (0 = Unknown/Other, 1 = MVT (PBF Vector Tile), 2 = PNG, 3 = JPEG, 4 = WEBP | 1 |
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| 100 | min zoom | 1 |
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| 101 | max zoom | 1 |
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| 102 | min longitude (signed 32-bit integer: longitude * 10,000,000) | 4 |
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| 106 | min latitude | 4 |
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| 110 | max longitude | 4 |
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| 114 | max latitude | 4 |
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| 118 | center zoom | 1 |
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| 119 | center longitude | 4 |
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| 123 | center latitude | 4 |
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### Notes
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* **# of addressed tiles**: the total number of tiles before run-length encoding, i.e. `Sum(RunLength)` over all entries.
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* **# of tile entries**: the total number of entries across all directories where `RunLength > 0`.
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* **# # of tile contents**: the number of referenced blobs in the tile section, or the unique # of offsets. If the archive is completely deduplicated, this is equal to the # of unique tile contents. If there is no deduplication, this is equal to the number of tile entries above.
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* **boolean clustered flag**: if `True`, blobs in the data section are ordered by Hilbert `TileId`. When writing with deduplication, this means that offsets are either contiguous with the previous offset+length, or refer to a lesser offset.
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* **compression enum**: Mandatory, tells the client how to decompress contents as well as provide correct `Content-Encoding` headers to browsers.
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* **tile type**: A hint as to the tile contents. Clients and proxies may use this to:
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* Automatically determine a visualization method
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* provide a conventional MIME type HTTP `Content-Type` header
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* Enforce a canonical file path extension e.g. `.mvt`, `png`, `jpeg`, `.webp`
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### Organization
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In most cases, the archive should be in the order `Header`, Root Directory, JSON Metadata, Leaf Directories, Tile Data. It is possible to relocate sections other than `Header` arbitrarily, but no current writers/readers take advantage of this.
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